In pressurized water reactors, the nuclear fuel is in the form of rods, which are held by grid spacers containing springs which press the rods against other springs or against dimples. A plurality of these grid spacers is positioned along the length of the rods, which may be ten or twelve feet. The rods extend between, but ordinarily do not penetrate, upper and lower tie plates. These plates and the grid spacers are attached to tie rods which usually also serve as guide tubes for the control rods which vary the reactivity of the assembly. The tie rods are often secured to the tie plates by quick release mechanisms which permit disassembly of the fuel assembly. The grid spacers are positioned at intervals along the length of the rods, with the lowest usually being several inches above the lower tie plate. The fuel rods expand several inches in length during their irradiation and the location of the upper- and lowermost grid spacers must take this into account.
One of the problems in connection with the operation of nuclear reactors is the accumulation of debris of various types, which may occur during the original construction or during repair and may include nuts, bolts, metal turnings, shavings, and various kinds of trash. The upper and lower tie plates are provided with comparatively large openings for the flow of water. The largest debris, which will not pass through these openings, causes no particular trouble. The smallest, which may be, for example, of the size of ordinary sand grains, passes through the assembly, again without causing any particular difficulty. The intermediate size debris, however, can be extremely troublesome since it may be caught between the fuel rods, and abrade them. This can cause escape of fuel and fission products into the cooling water, creating radiation problems. The tendency to lodge between fuel rods is particularly prevalent in the portion of the rods below the lowest grid spacers.
One of the attempts to mitigate the debris problem has been to use very long end caps, which extend from the ends of the fuel rods to the lowermost grid. Thus, the portion of the rod which is most exposed to abrasion by the debris does not contain fuel, and therefore does not give rise to the problem of radiation escaping as described above. However, this has the disadvantage of replacing a portion of the fuel by inert material, and thus reducing the available power of the assembly. Another proposed solution has been the provision of small holes in the lower tie plate which act as a screen and to prevent the debris from contacting the fuel rods. However, this increases the pressure drop through the assembly and can result in a lower coolant flow and, therefore, lowered permissible power level. Moreover, the debris which does not pass through the lower tie plate is held there and, when the circulation of the cooling water is discontinued, drops to the bottom of the reactor vessel where it is available to cause trouble the next time the circulation is started.